Electrical heating element and method of manufacture



Ap 25, 1933. A. J. KERCHER El AL 1,905,232

ELECTRICAL HEATING ELEMENT AND METHOD OF MANUFACTURE Filed Dec. 17, 1928 IIXV NTORS.

NE YS.

Patented Apr. 25, 1933 UNITED STATES PATENT OFFICE ARTHUR J. KERCHER, OF BERKELEY, AND WILLIAM WESLEY HICKS, OF SAN FRANCIS,

CALIFORNIA, ASSIGNORS TO WESIX NATIONAL COMPANY, OF SAN FRANCISCO, CALI- FORNIA, A CORPORATION OF DELAWARE ELECTRICAL HEATING ELEMENT AND METHOD OF MANUFACTURE Application filed December 17, 1928. Serial No. 326,531.

This invention relates generally to electrical heating elements such as are used in electrical appliances, and to methods of manufacturing the same.

It is an object of this invention to devisean electrical heatin element which is simple in construction, an which will give eflicient service over relatively long periods of use.

It is a further object of this invention to devise an electrical heating element which will have relatively small heat lag and which will incorporate adequate means for insulating the convolutions of the resistance conductor.

It is a further object of this invention to devise a novel process for manufacturing electrical heating elements, which process requires no elaborate apparatus and is simple to apply.

It is a further object of this invention to devise a novel method of manufacturing electrical heating elements which will make possible adequate insulation of the resistance conductor without the necessity of embedding the conductor within an insulating medium.

Further objects of the invention will appear from the following description in which we have set forth the preferred embodiment of our invention. It is to be understood that the appended claims are to be accorded a range of equivalents consistent with the state of the prior art.

Electrical heating elements such as are commonly used in electrical appliances generally consist of a metal sheath within which is disposed one or more electrical resistance conductors. Various expedients have been devised for adequately insulating the resistance conductors from electrical contact with the metal sheath, and for insulating the convolutions of the conductor from direct con tact with each other. For example in one type of prior heating element the convoluted resistance conductor is embedded in powdered refractory insulating material, which is tightly packed within the metal sheath. Heating elements constructed in this manner not only have a relatively large heat lag due to the relatively large amount of refractory insulating material employed, but failure of these elements is comparatively common due presumably to difficulties in securing a uniform product by the manufacturing methods employed and to un-uniformities which are apt to occur in the material itself. With our invention adequate insulation is provided for the resistance conductor without entirely embedding the same and the method of manufacture which we employ assures a comparatively uniform product.

Referring to the drawing for a detailed description in Figs. 1 and 2 we have shown a typical form of electrical heating element which includes generally a metal sheath 10, in this instance in the form of a tube, within which are positioned a plurality of tubular refractory insulating tubes 11. Tubes 11 are arranged within the sheath 10 in contiguous or end to end relationship, and disposed within these tubes there is a resistance conductor 12, this conductor being preferably convoluted in the form of a helix. The tubes 11 are made of some material which not only has relatively high di-electric strength but also is a fairly good conductorof heat. In practice we have secured good results by utilizing tubes made of alundum. In the manufacture of these tubes the powdered alundum is mixed with some suitable binder such as fire clay, and this mixture in moist condition is formed into the tubes. The tubes are then dried and vitrified in a suitable kiln. Tubes of this character have high dielectric strength which will not break down at the relatively high temperatures to which they are subjected.

As shown in the enlargement of Fig. 3, the tubes 11 have a fairly close fit within the sheath 10, so that heat is effectively transferred from these tubes to the sheath. Likewise the helical resistance conductor 12 has a fairly close fit with the inner walls of the tubes 11, so that aneffective heat conducting path is provided from the resistance conductor, thru the tubes, to the sheath. One end of each tube is preferably beveled inwardly as indicated at 13 so as to receive a projecting beveled portion 14 provided u on the end of the next adjacent tube. In ot er words each tube has one inwardly beveled and one forwardly beveled end.

In connection with the assembly described above, we preferably provide adqeuate insulation to prevent short-circuiting between the convolutions of the resistance conductor 12. The nature of the preferred form of this insulation can best be understood by first describing a preferred method of constructing our heating element.

As shown in Fig. 4 we have found it convenient to first assemble the refractory tubes 11 u on a length of the convoluted resistance con uctor 12. As indicated in Fig. 4, while this assembly is held in a general y upright position, a suitable fluid cement is introduced mto the upper end, preferably into the hollow space provided by the interior of the helical conductor 12. The tubes 11 at this time preferably have their forwardly beveled portions 14 directed downwardly to minimize seepage between the tubes. The cement which is used must have adequate insulating properties and at the same time must be refractory in nature to withstand the high temperatures employed. It is preferably rather viscous so that while assing down through the assembly, it will ow over and coat the resistance conductors and accumulate between the convolutions.

We have found that suitable cement can be formed by a mixture of powdered alundum and water, to which can be added a suitable binder such as fire clay. Magnesium oxide or like material can be used in place of alundum. These materials are formed into a wash which is sufiiciently fluid to flow down thru the helical conductor and thru the insulating tubes, but which has suflicient consistency and viscosity to cling to and coat the conductor and accumulate between the conductor convolutions. We prefer that an excess of this material be poured into the upper end of the assembly as indicated in Fig. 4, and that the excess cement be permitted to drain from the lower end. By this method we secure adequate distribution of the cement over all portions of the inner walls of insulating tubes 11 and over the convolutions of the resistance conductor. The treated assembly is then permitted to dry preferably before being inserted within a sheath and before it is put in use. This drying operation does not require a baking oven, since the interior is open to the atmos here and since the amount of cement is re atively small. After drying, the assembly is slid into the metal iheath 11 to form the assembly shown in e manner in which the cement is disposed between the convolutions of the resistance conductor in order to insulate these convolutions from direct electrical contact with each other is clearly shown in Fig. 3, in which this cementitious material has been indicated at 16. Since some of this cement is also in contact with the inner walls of insulating tubes 11 it serves to form a more effective heat transferring relationship between the conductor and these tubes.

When the completed heating element is initially put into use, the cement 16 may be further baked or may even be vitrified by the operating temperature. Suflicient amount of this cement is provided so that it generally prevents displacement of the resistance conductor, or even if a certain amount of displacement should take place, it prevents direct electrical contact between the convolutions. With the construction described the completed unit is relatively li ht in weight and has comparatively small eat lag. In other words it will quickl reach its desired operating temperature a ter the currentis applied.

t is also characteristic of our construction and our method of manufacture that the fluid cement is retained within the insulating tubes 11 and does not come directly in contact with the metal sheaths 10, except in the event that a comparatively small amount of this material may seep between the tubes at their point of junction. Therefore the cement which we employ need not be of an expensive high grade, since the resistance conductor would not be grounded to the metal sheath even though the cement may have substantially less resistivity than the insulating tubes.

As shown in Fig. 5, the cement can also be applied after the assembly of Fig. 4 has been inserted in the sheath 10. However it has been found preferable to apply the cement before the inserting of the assembly within the sheath, since it is simpler and more convenient to dry the unsheathed assembly.

This case is a continuation in part of application 136,444, filed September 20, 1926.

We claim:

1. The method of constructing an electrical heating element making use of a convoluted resistance conductor strung thru refractory insulating tubes, said method comprising forming an assembly of the tubes upon the conductor, introducing fluid cement into one end of the assembly while the assembly is retained in a general upright position, and permittin to set only that portion of the cement which adheres naturally to the interior surfaces to which it is applied.

2. The method of constructing an electrical heating element making use of a helicoidal resistance conductor together with tubular refractory insulating tubes and a tubular metal sheath, said method com rising forming an assembly of the tubes with the conductor extended thru the same, introducing viscous fluid cement into the upper end of the assembly when the assembly is in a general upright position, permitting free drain e of excess cement from the lower end 0 the assembl whereby the helicoid formed by the con uctor is comparatively hollow, drying the cement, and inserting the assembly within the sheath to leave a space within the conductor and permitting the cement which remains to set.

3. An electrical heating element comprising a metal sheath, a plurality of heat transmitting tubes of refractory insulatin material disposed within the sheath and in heat transferring relationship with the same, said tubes having interfitting end portions, :1 helicoidal resistance conductor strung thru said tubes and in heat transferring relationship with the same, and refractory cement disposed between the convolutions of the conductor, the interior of the helicoid being practically unfilled with said cement.

4. The method of constructin an electrical heating element makin use 0 a convoluted resistance conductor an a plurality of refractory insulating tubes, said method comprising stringing the insulating tubes upon said conductor, applying refractory cement in fluid condition to the convolutions of the conductor and the adjacent surfaces of the insulating tubes, permitting excess cement to drain from the assembly to leave a space within the conductor and permitting the cement which remains to set.

5. The method of constructing an electrical heating element making use of a helical resistance conductor and refractory tubular insulation which method comprises inserting the helical conductor into the bore of the tubular insulation, pouring fluid insulating cement into the bore to cover the resistance conductor with an adherent coating ofthe cement, allowing the portion of the cement which does not adhere to drain off and leave a restricted bore through the heating element, and then causing the adherent coating of cement to harden.

In testimony whereof, we have hereunto set our hands.

ARTHUR J. KERCHER. WILLIAM WESLEY HICKS. 

